Automatic billet inspector

ABSTRACT

METAL BILLETS ARE CONVEYED PAST A DEFECT SCANNING UNIT THAT RECIPROCATES SEARCH COILS ACROSS A BILLET SURFACE. THE SCANNING UNIT IS MOVABLE TO A POSITION ADJACENT THE BILLET SURFACE AND THE SEARCH COILS ARE INDEPENDENTLY POSITIONED ON THE BILLET SURFACE. SCANNING UNIT MOVEMENT ACROSS THE BILLET IS REVERSED IN RESPONSE TO SENSING OF BILLET EDGES.

J. J. SKU B IAK ET AL AUTOMATIC BILLET INSPECTOR Jan. 5, 1971 12 Sheets-Sheet 1 Original Filed Nov. 1, 1.965

INVENTORS John J. Skubiak John A. Tof'h ATTORNEYS Jan. 5, 1971 .1. J. SKUBIAK ETAL 3,553,570

AUTOMATIC BILLET INSPECTOR Original Filed Nov. 1, 1965 13 Sheets-Sheet 2 I c; 8 I Q J: q ,0 r- Q0 0 T I k 5 R F Fen w T: m F I 0 Q 03 g I: J H g! v i g 1':

LL mg Q to Q INVENTORS John J. Skubiak John A. Tofh ATTORNEYS.

Jan. 5, 1971. J. J. SKUBlAK- 3,553,570

AUTOMATIC BILLET INSPECTOR Original Filed Nov. 1, 1965 12 Sheets-Sheet 3 iNVE/VTORS John J. Skubiak John A. Tofh ATTDNYS .1971 J. J. SKUBIAK ET AL 3,553,570

AUTOMATIC BILLET INSPECTOR I2 Sheets-Sheet 4 Original Filed Nov. 1, 1965 T I84 112 157 1/5 I85 INVENTORS John J Skubiak HOP INF"

John A Tofh ATTORNEYS Jan. 5, 1971 J y s l K ET AL 3,553,570

AUTOMATIC BILLET INSPECTOR Original Filed Nov. 1, 1965 12 Sheets-Sheet 5 INVENTORS John J. Skubiak John A. Toih ATTORNEYS Jan. 5, 1971 SKUBlAK ETAL 3,553,570

' AUTOMATIC BILLET INSPECTOR Original F iledNoV. 1, 1965 12 Sheets-Sheet e INVENTORS John J. Skubiok John A. Tofh ATTORNEYS.

. 5, 1971 J. J. SKUBIAK ETAL 3,553,570

AUTOMAT I C BILLET INSPECTOR Original Filed Nov. 1, 1955 12 Sheets-Sheet 1o 256 NO! HEAD 250 8 N02 HEAD OSCILLATOR COL Com T OSCILLATOR UNIT 252 UNIT osclLLArm COM 253/ Com OSCILLATOR UNIT UNIT 257 /260 LOW FREQUENCY UNIT NO! HEAD ,H,. LOW FREQUENCY UNIT No.2 HEAD 26I- MARKER PANEL NO.I HEAD MARKER PANEL M N02 HEAD POWER SUPPLY 270 264 I E DAIER- NO.I HEAD A E LXI VALVE-AIR L BOTTOM MARKER r266 VALVE AIR TOP MARKER w MERCURY N02 HEAD RELAY VALVE AIR L wrm'w MARKER 269 INVENTORS John J. SIwDIak John A. ToIh DY W Jan. 5, 1971 J, SKUBIAK ET AL 3,553,570

AUTOMATIC B ILLET INSPECTOR Original Filed Nov. 1. 1965 L2 Sheets-Sheet l1 1 (86 w P IL 28Oa 275 85\ @290 SOIJ3 2891') INVENTORS John J. Skubiak John A. Towh ATTORNEYS J. J. SKUBIAK ET AL 3,553,570

AUTOMATIC BILLET INSPECTOR l2 Sheets-Sheet 13 Original Filed Nov. 1, 1965 All \Em mm @m N mm 5w 3 w 3 w m @m r h fU Wm 9m L I WA II I .llll wwm mmm mmm mmm wmmv 0mm n 0 .wmm W P u 4% E mm? 8m 1 vm E8 J a #VIH vvm mi 3 Q? I 5% w: IIIJ lllllll l1 mmm 3,553,570 AUTOMATIC BILLET INSPECTOR H John-{ J. Skubiak, Madison Heights, Mich., and John A.

Tofh, Highland Heights, Ohio, assignors to Republic Steel Corporation, Cleveland, Ohio, a corporation of New Jersey Original application Nov. 1, 1965, Ser. No. 505,940.

Divided and this application Jan. 22, 1969, Ser.

Int. Cl. G011 33/12 U.S. Cl. 324-37 13 Claims ABSTRACT OF THE DISCLOSURE Metal billets are conveyed past a defect scanning unit that reciprocates search coils across a billet surface. The scanning unit is movable to a position adjacent the billet surface and the search coils are independently positioned on the billet surface. Scanning unit movement across the billet is reversed in response to sensing of billet edges.

This application is a divisional application of Ser. No. 505,940, filed Nov. 1, 1965.

This invention relates to the inspection of metal objects, and more especially steel billets, particularly for detecting and evaluating surface defects in steel billets.

The surface characteristics of steel billets are such that the detection of defects is difiicult. Superficial cracks may be almost, if not entirely, invisible to the naked eye Moreover, the surface appearance of a defect often affords no indication of the depth and, there-fore, the significance of the defect.

Methods and apparatus for detecting the presence of flaws or imperfections of the type referred to above by measurements conducted at the surface of a metal test piece are disclosed in U.S. Pat. No. 2,660,704 issued to William C. Harmon et al.; and in U.S. Pats. Nos.'i2,'8'32,- 040; 2,914,726; and 2,979,196 issued to William C. Harmon. In the apparatus disclosed in the above patents, a high frequency oscillator is utilized to energize azsearch coil. The search coil generates an electromagnetic field. When the search coil is placed adjacent a billet or other metallic object to be tested, the reaction produced: 6n the search coil by the interaction of the test object the electromagnetic field modifies the operation of high frequency oscillator in accordance with the flawin the metal of the test piece. The effect of the metal object being tested upon the performance of the oscillator as the search coil is moved relative to the surface of the object thus provides a measure of the presence or absence of flaws in the test object.

Scams and other defects must be detected and removed from steel billets before the billets are rolled so that the resulting products are not defective. Once the defects are located, they are removed by grinding or scarfing the billet in the area of the seam. The sea-m or other defect must not only be located, but also the severity of the defect must be evaluated so that the worker scarfing the billet need not waste time on defects that appear to be serious, but are actually only superficial blemishes.

Several characteristics of steel billets make them extremely difficult to inspect automatically. These include, (1) the billets are made in a wide variety of lengths and cross sectional areas, (2) frequently the billets are bent or twisted longitudinally, (3) when the billets arecut off, jagged stiff projections known as burrs or snags often remain on the ends of the billets and (4) steel slivers often project from surfaces of the billet intermediate the ends. These projections interfere with both billet feeding and scanning. Accordingly, it has been the general practice to inspect such articles visually or occasionally with tes Patent 3,555,570 Ice Patented Jan. 5, 1971 portable detectors having a search coil or probe that is moved by hand over the surface of the object. Such methods are, of course, slow and inefficient. They often result in scarfing of superficial blemishes with the ap pearance of serious defects. Conversely, and more importantly, serious defects are often overlooked.

In accordance with the present invention, methods and apparatus are provided to facilitate the automatic han dling and inspecting of large steel billets of dilferent sizes. An arrangement is provided so that two surfaces of rectangular shaped billets may be simultaneously scanned. In addition, two inspection assemblies are ar-= ranged in sequence, to automatically inspect the four major surfaces of such billets.

Billets to be inspected in accordance with this invention are usually first grit-blasted to provide a clean, scalefree surface to permit close inspection tolerances, accurate indications of changes in the electromagnetic field used for detecting defects, and toi facilitate the adherence of material, such as paint, used tomark the presence of any defects detected. After the initial preparation, the billets are conveyed along a predetermined horizontal path past a first inspection station. The billet is conveyed by V-rollers so that two adjacent longitudinal surfaces of the billet face upward, each at a 45 degree angle from the horizontal. Two scanning heads are provided at the inspection station, one for each upper surface of the billet, so that two surfaces of the billet may be simultaneously inspected. A mounting arm is located on each side of the path along which the billet is conveyed and supports one of the twoscanning heads. Each arm extends at essentially a 45 degree angle from the horizontal and may be pivoted toward and away from a position adjacent the billet.'A suitable parallel linkage associated with the arm and scanning head maintains the head parallel to the surface'of the billet when the supporting arm is pivoted.

Each scanning head includesIa pair of twin search coils for sensing defects, two spaced proximity coils for de-= tecting the longitudinally extending edges formed by ad= jacent surfaces of the billet, and a pair of markers for indicating the'presence and location of any defect de tected in the billet. The scanning heads are mounted on the arms for independent movement transversely of the billet The search coils and proximity coils are supported by the scanning unit for independent movement into and out of direct contact with the billet once the mounting arms are lowered into a position where the scanning heads are adjacent to the billet. Appropriatetiming of this independent movement protects the 'lcoils fromthe burrs or snags that extend from the ends of the billet and otherwise'sometimes catch the search unit and destroy part or all of the unit.

A control system is provided to automatically initiate and control the three modes of movement of each of the scanning heads in response to the presence, location and size of the billet. Two light beams for each scanning head are directed across the predetermined path along which the billet is conveyed and are received by two photoelectric cells. When the billet intercepts thev beams, its presence at the inspection station is sensed. As a result, at the appropriate time the control system causes the mounting arms suporting the scanning heads to rapidly move transversely "of the billet to position each scanning head adjacent one of the two upward-facing billet sur balancing force to be applied to the mounting arm to diminish the force with which the arm is held in an inspecting position against the surface of the billet. Rollers carried by the mounting ar-ms adjacent each scanning head roll on the surface of the billet being inspected to insure that the scanning head is properly located with respect to the surface being inspected. Thus, firm contact is maintained even if the billet is longitudinally bowed or has an uneven surface. The further actuation of the cam lever, above mentioned, plus the interruption by the billet of a third light beam associated with each scanning head initiates a lowering of the search coils of the scanning head to the billet. This occurs only after the end of the billet has moved beyond the inspection station so that the scanning unit will not be damaged by the end of the billet, which may extend out of the desired path if'the billet is bowed or which may carry burrs or snags that could catch the search coil unit.

With the scanning head and search coils in proper position adjacent the surface of the billet to be inspected, the scanning head is automatically reciprocated relative to both the mounting arm and the billet, in a direction transverse to the billet movement. The distance that the scanning head is moved in the transverse direction is controlled by an upper and lower proximity coil on each side of the scanning coils. When the scanning head approaches either the upper or lower lateral edge of the billet surface during reciprocation, one of the proximity coils is moved beyond the edge of the billet. This condition is sensed electrically and the direction of movement is reversed.

The longitudinal movement of the billet and the transverse reciprocating movement of the scanning heads create a zigzag scanning path of each pair of search coils across the respective upward-facing surface of the billet. Paint markers carried by each scanning head are positioned with respect to the search coils so that they follow the same path over the billet just behind the search coils. The paint markers are triggered when the search coils sense a defect of a predetermined minimum severity and deposit a paint mark upon the billet directly over the location where the defect was detected. The marking system is preferably of the type described and claimed in US. patent application Ser. -No. 287,987 filed June 14, 1963 by Joseph M. Mandula et al. under the title High Speed Marking System," now patent No. 3,418,567.

When the billet leavesthe inspecting station, the light beams are no longer interrupted, and the inspection stops. The mounting arms are automatically withdrawn to the original position, ready for the next billet.

Defects are detected by the search coils of the scanning head. The coils, as suggested above, establish an electromagnetic field in the article being tested and also detect variations in the .field caused by flaws in the article. The search coils themselves need not actually touch the article beingjnspected, but should be closely spaced therefrom. They may be properly located with respect to the article surface by a wear plate interposed between the coils and the article and adapted to slide on the surface of the article. The search coils are energized by an oscillating vacuum tube and generate currents in the steel or other metal being tested. When a flaw in the metal is encountered, the loading of the search coils is decreased and therefore the output of the oscillator increases. This variation in oscillator output is detected by an electrical system.

Another feature of this invention is the provision of a built-in calibration surface that may be selectively positioned in a location at a billet inspecting station. The scanning heads may be then located adjacent the calibrating surface and the sensitivity of the search coils adjusted so that only those seams that exceed a preset depth or severity will be marked during the inspection.

The present invention is applicable to a large range of sizes of billets without necessitating adjustment of the apparatus for each individual size, it is capable of greatly reducing the handling costs directly related to billet inspec- CJI tion, and it reduces unnecessary subsequent conditioning of billets where only minor or superficial defects exist. By efficiently, accurately and automatically detecting and marking objectional defects of predetermined severity in steel billets, the present invention greatly reduces or eliminates the expensive and wasteful processing of defective material into finished or semi-finished parts.

Other attendant advantages and the features of this invention will be readily appreciated as the same become better understood by reference to the following detailed description, when considered in connection with the accompanying drawings in which:

FIG. 1 is a top plan view showing the overall arrangement of two inspection stations constructed and arranged in accordance with the present invention for automatically inspecting steel billets;

FIG. 2 is an end elevational view of the inspection station shown at the right side of FIG. 1, showing two scanning units and support arms of the inspection station in position to inspect a billet;

FIG. 3 is a partial side elevational view of the inspection station of FIG. 2, with parts removed, showing a calibrating mechanism used in conjunction with the inspection station;

FIG. 4 is an end elevational view of the calibrating structure shown in FIG. 3;

FIG. 5 is a diagrammatic perspective view of one of the scanning units of FIG. 2, showing details of the main roller, sensing roller and the housing for the sensing coils FIG. 6 is a side elevational detailed view of the scanning unit, showing the mounting means and means for reciprocating the scanning unit;

FIG. 7 is a fragmentary top plan view of the search unit subassembly of the scanning unit, which can be independently lowered into contact with a billet and which senses defects and produces signals for controlling the reciprocation of the scanning unit;

FIG. 8 is a side elevational view of the subassembly of FIG. 7;

FIG. 9 is a sectional view of the sensing coil housing, taken along the lines 99 of FIG. 7 and showing details of the proxii'nity coils and associated structure;

FIG. 10 is an end elevational view of the sensing coil housing of FIG. 7;

FIG. 11 is a bottom plan view of the sensing coil housing of FIG. 7, showing details of the defect sensing coil;

FIG. 12 is a wiring diagram of the control circuit for operating an inspecting station, including raising and lowering the scanning unit and search coils relative to a billet;

FIG. 13 is a wiring diagram of the calibration control circuit associated with the control circuit of FIG. 12, for raising and lowering the calibration bar and for controlling the operation of the scanning unit during calibration;

FIG. 14 is a wiring diagram of a circuit associated with the proximity coils of a scanning head for controlling the reciprocation of the scanning unit across the surface of a billet being inspected and for selecting the proper defect marker to be actuated during the reciprocation of the scanning unit;

FIG. 15 is a block diagram of a signalling circuit illustrating the manner in which the sensing coils produce signals which trigger the defect markers to indicate the presence of a defect directly upon the billet;

FIG. 16 is a diagrammatic layout of the hydraulic system forraising and lowering the scanning units and for reciprocating the scanning units across the surface of the billet being inspected;

FIG. 17 is a diagrammatic layout of the pneumatic system for raising and lowering the sensing coils relative to the billet and for raising and lowering the calibration bar, as well as for operating the defect markers; and

FIG. 18 is a fragmentary view of a surface of a billet of the type inspected by the apparatus disclosed herein, diagrammatically showing the scanning path followed by the sensing coils of a scanning unit as the billet is moved longitudinally through the inspection station.

(I) GENERAL MECHANICAL ARRANGEMENT The general arrangement of two inspecting stations constructed in accordance with this invention and arranged to sequentially inspect adjacent pairs of billet surfaces is shown in FIG. 1 of the drawings. First and second inspection stations are indicated generally by the reference numerals 2t and 22, respectively. In the arrangement shown, the two inspectionstations are in side by side relationship; the inspecting station 20 receives a billet moving in the direction of. the arrow along a predetermined horizontal path P After a billet has passed completely through the station 20 along path P it is turned over by a suitable mechanism, not shown. The billet is then fed into the second inspection station 22. The billet moves in the opposite direction through the second station 22 as shown by the arrow along a predetermined horizontal path P A billet -B is shown in phantom in the station 22. Because the mechanisms providing the two inspection stations are identical, only mechanism providing the second inspection station 22 will be described in detail. It will'be understood, ofv course, that all four sides could be scanned at one station or successive stations without turning the billet over by providing two additional arms that move into position adjacent the lower surfaces of the billet.

Two conveying rolls 24, 25 are longitudinally spaced along the path P Each conveying roll 24, 25 rotates about a horizontal axis that extends at right angles to the path of travel P The roll 24 is supported for rotation upon a horizontal shaft 28 supported at each end by journal bearings 29 and 30. Similarly, the conveying roll 25 is supported by a horizontal shaft 32 supported at each end by journal bearings 33 and 34. One or both of the conveying rolls may be driven. In the embodiment shown, the roll 25 is driven through a gear box 36 having an output shaft 37 coupled to' the supporting shaft 32, and conveying roll 24 is an idler roll. In contrast, both rolls shown in the arrangement of the first inspection 20 are driven by gear boves 36a, 36b respectively.

Thetwo conveying rolls 24, 25 are constructed with a V-shaped periphery so as to have two surfaces 39, 40 on the roll 24 and 42, 43 on. the roll 25 positioned at right angles to each other to receive a longitudinally extending edge or corner of a square" or rectangular billet.

Two inspection assemblies 45, 46 are located between the two conveying rolls 24,v 25 and extend across the path P As more clearly shownin FIG. 2, the inspection assemblies 45, 46 each consist essentially of a support arm 49, 50 and an attached housing 53, 54, respectively, which carry units for scanning and inspecting a steel billet. Each arm 49, 50 is pivotally supported at a lower end upon a mounting bracket 55, 56, respectively. The mounting brackets are supported on opposite sides of the path P by a base housing, indicated generally at 58. The arms 49, 50 may be pivoted about the mounting brackets toward and away from the path P in the area between the two conveying rolls 24, 25. In this manner, the arms bring the billet scanning units carried by the housings 53, 54 into position to inspect the billet and then out of position to receive a new billet.

Various control elements are located at the inspection stations, including three photo electric cells and light sources for each scanning unit. In FIG. 3, three cells 60, 61, 62 for the inspection assembly 45 are digarammatically shown in alignment along the billet path. They are mounted in a housing 63 on a bracket 64. These cells 'detect the presence and location of the billet to be-inspected. Additional control elements are carried bythe housings 53, 54, and will be described'in more detail subsequently.

A calibrating bar and actuating mechanism, as shown in FIGS. 3 and 4, and indicated generally by the reference numeral 65', is associated with each inspection station 20, 22 and is located between the two conveying rolls, just beneath the path of travel of the billet. As will be explained in more detail subsequently, before the inspection of a billet, the calibrating bar may be raised at the inspecting station to a position occupied by a billet when the device is in use. The inspection assemblies may then be lowered and calibrated to a desired degree of sensitivity. The calibrating bar is then lowered, and a billet or series of billets inspected.

(II) THE SCANNING ASSEMBLIES (A) Support arms Reference is made particularly to FIGS. 1 and 2 of the drawings. Because both inspection assemblies 45 and 46 are identical but reversed, only the inspection assembly 45 will be described in detail.

The main support arm 49 pivoted on the mounting bracket 55 supports a housing bracket 68 at its distal end. Two, spaced, parallel arm members 78, 71 extend upward at an angle from each side of the mounting bracket 55 toward the path P When the arm members 70, 71 are in a lowered position to inspect a billet, asshown in FIG. 2, they extend at approximately a 45 degree angle from the horizontal. A pair of parallel links 74, 75 are each associated with a corresponding one of the spaced arm members 70, 71. The links extend parallel to and above the arm members between the mounting bracket 55 and the housing bracket 68. The parallel links 74, 75 and the spaced arm members 70, 71 are pivotally'secured to both the mounting bracket 55 and the housing bracket 68. Together, the arm members, the links and the brackets 55,

68 form a parallelogram linkage. This linkage maintains the bottom of the housing 53 carried by the housing bracket 68 at a 45 degree. angle with the horizontal, regardless of the angular position of the spaced arm members 70, 71.

The spaced arm members 70, 71 are supported on the mounting bracket 55 by a rotatable shaft 78 and are secured in fixed relationship with the shaft 78 by three, wire-locked, screws 80. A counter weight, 82, in the general form of a bell crank, is carried by the rotatable shaft 78 and keyed or otherwise secured for rotation with the shaft. As best seen from FIG. 2, the counter weight 82 extends from the mounting bracket 55 at an obtuse angle from the arm 49 to counter balance the moment of the support arm 49. A lower extending portion 82a of the counterweight 82 acts as a lever arm, to pivot the counterweight 82 in the manner of a bell crank about the axis of rotation of the rotatable shaft 78. Such rotation of the counterweight 82 is effected by a hydraulic cylinder 86 fastened to the base member 58. A piston rod 87 of the cylinder 86 is connected to the lower extending portion 82a of the counterweighLI-Iydraulic fluid under pressure is supplied to the hydraulic cylinder 86 and other units of a hydraulic system used to operate the inspection assembly by variable volume pumps VPl, VPZ driven by an electric motor PMl, all associated with the. base 58.

Pivotal movement of the support arm 49 about the axis of rotation of the rotatable shaft 78 in a direction away from the path P is limited by a mechanical stop element on the base 58-. The stop 95 cooperates with a block 96 carried by the counterweight 82. Downward movement of the support arm 49 is normally limited by a steel billet carried by the conveying rolls 24, 25. A safety switch 18-9 is carried by the arm 49, which switch is actuated by a stop (not shown) in the event the arm 49 is lowered in the absence of a billet.-

A main roller 99 is supported across the lower surface of the housing 53, FIGS. 2 and 5. The main roller 99 has an axis of rotation extending parallel to the bottom of the housing 53 and transversely of the direction of the billet movement. Journals 100 at each end of the roller 99 are supported for rotation in bearings 101, 102 carried by the housing 53. The roller 99 is adapted to ride upon the surface of the billet being inspected when the support arm 49 is in lowered position.

A limit switch actuator comprised of a sensingroller =103 carried on a lever arm 104! (see FIG. is pivoted about a journal 100 and extends outwardly from housing 53 and roller 99. The sensing roller is carried with the arm 49 and, as the arm approaches the scanning position, the sensing roller contacts the surface of the billet before the main roller 99. Billet contact by the sensing roller rotates the lever arm 104 and operates a limit switch in the control circuit. As will be described in more detail, actuation of the limit switch reduces the speed at which the arm 49' approaches the billet. Further rotation of the lever arm 104, as the arm 49 carries the roller 99 into contact with the billet, actuates a second limit switch in the control circuit to apply a counter force in the hydraulic actuator 86 to reduce the contact pressure between the main roller 99 and the billet from that pressure utilized to move the arm 49 into position. Thus, the arm is maintained in scanning position under a reduced force, with the roller 99 riding on the surface being inspected. This maintains the scanning unit at a fixed distance from the surface of the billet, even though the surface is not uniform.

(B) Scanning unit As best shown in FIGS. 2 and 6, a scanning unit, indicated generally by reference numeral 105, is carried by the housing 53 of the support arm 49. A similar scanning unit is carried by the housing 54 of support arm 50 and, being the same as the other, will not be described in detail.

The scanning unit 105 is supported by the housing 53 for reciprocation transversely of the billet movement. The unit includes a subassembly 107, which is separately movable into and out of direct contact with a billet after the housing 53 is positioned over the billet by the arm 49. See FIGS. 7-11. The subassembly includes twin search coils 110, 111 that detect defects in the billet, and proximity coils 112, 113 that signal a reversal of the scanning unit movement at the edges of the billet. Two paint markers 114, 115 are carried by the scanning unit for reciprocation along with search coils to mark the presence and location of any defects. An air cylinder 116 is carried to actuate the subassembly into and out of contact with the billet.

The housing 53 supports a cylinder rod 118 extending in a direction transversely of the path of billet movement. The rod is tubularv and supported at each end by supports 120, 121 of the housing. A movable hydraulic cylinder 122 surrounds the fixed cylinder rod 118 and reciprocates along the rod. A stationary piston 124 is fixed to the cylinder rod intermediate the ends of the rod and within the movable cylinder 122. The rod is divided by a baffie or is otherwise plugged at the plane of the piston to isolate the end of the rod on one side of the piston from the other end.

Hydraulic fluid lines, shown schematically in FIG. 16, which will be described in more detail subsequently, are connected to each end of the cylinder rod 118. Openings 125 are located in the wall of the cylinder rod on each side of the piston. With this arrangement, fluid introduced into one end of the cylinder rod 118 fills the cylinder on that side of the piston, thereby moving the cylinder toward the end of the cylinder rod through which the fluid is introduced. At the same time, fluid on the other side of the piston in the cylinder 122 escapes into the cylinder rod 118 and out the opposite end of the rod. A control valve 126 is supported by the housing 53 and controls the direction in which hydraulic fluid is introduced into the cylinder 122. The manner in which the valve 126 is actuated will be described in more detail subsequently in connection with the control system of the invention.

A front end cap 127 and rear end cap 128 are provided on the cylinder 122 to closely encircle the cylinder rod 118. The front end cap 127 carries an adapter 129 that receives a guide rail 130, which is fixed to the housing 53. The guide rail extends parallel to the cylinder rod 118 and maintains the cylinder in proper alignment, preventing the cylinder from rotating about the rod. A pair of spaced brackets 132, 133 are secured to the adapter 129. The brackets attach the subassembly 107 and the actuating air cylinder 116 to the movable hydraulic cylinder 122 for reciprocation relative to the billet.

As best shown in FIGS. 7 and 8, the bracket 132 includes a rearwardly extending arm 132a and a depending portion 13%. Bracket 133 is similarly formed and is located parallel to the bracket 132 on the opposite side of the cylinder 122. The air cylinder 116 is pivotally mounted between the rearwardly extending arm portions 132a, 133a by journals 134, 135 at the forward end of the air cylinder.

A mounting bracket 136 is provided to secure a search coil housing 156 to the brackets 132, 133. The mounting bracket 136 has two rearwardly extending legs 138, 139 which are mounted to the brackets 132 and 133, respectively. Each leg 138, 139 is generally triangular in shape in side elevation, as best shown in FIG. 8, and a lower portion of the base of each triangular-shaped bracket is secured to the depending portion 132b, 133b, respectively, by a shaft 141. The shaft 141 permits pivotal move rnent of the mounting bracket 136 with respect to the brackets 132, 133. Two parallel spaced arms 144, 145 form the forwardly extending portion of the mounting bracket 136 and support the search coil housing 156.

A piston rod 148 extends from the front of the air cylinder 116 and is suitably secured to the cross piece of a clevis 150. Each extending arm of the clevis 150 is pivotally connected to the upper corner of the base of each triangular shaped leg 138, 139 of the mounting bracket 136. With this arrangement, actuation of the air cylinder pivots the mounting bracket 136 about the shaft 141 fastened to the brackets 132, 133, which are carried by the movable hydraulic cylinder 122. This moves the search coil housing 156 into and out of direct contact with the billet being inspected after the inspection assembly 45 has been lowered into scanning position, with the main roller 99 riding on the surface of the billet.

The housing 156 is of generally rectangular configuration, with an inclined lower edge 157 about the three sides that are leading edges with respect to the directions of relative movement between the billet and the housing. These edges flatten or ride over slivers on the surface of the billet which would otherwise catch the scanning unit. Lugs 15-8 and 159 extend upward from the housing, one along each of two opposite lateral sides, centrally of the housing. Bearings 162, 163 within the lugs 158, 159 receive shoulder screws 166, 167, which extend through the forward arms 144, 145 of the mounting bracket 136. In this manner, the housing 156 is pivotally supported at the forward portion of the mounting bracket 136.

Two twin search-coil assemblies 110, 111 are carried in a central portion of the housing, essentially flush with the bottom surface of the housing. The two twin search-coil assemblies 110, 111 are aligned transversely of the housing 156, so that their axis of alignment extends at right angles to the direction in which the housing 156 is reciprocated by the movable hydraulic cylinder 122. Each twin search-coil assembly is comprised of two search-coils, those coils of twin search-coil assembly 111 being indicated at 173 and 174. See FIG. 11. Each coil is formed of a large number of turns of relatively fine wire, wound into a fiat, circular form that might be termed a pancake coil, and the two coils of each twin search-coil assembly are electrically connected together in series opposition. Twin search-coil assemblies of the type c0ntemplated herein are disclosed in more detail in the copending application of William C. Harmon, Ser. No. 462,907 filed June 10, 1965 and entitled Method and Apparatus sssas' a for Inspecting Metallic Objects," now Pat. No. 3,497,799, the disclosure of which is hereby incorporated herein by reference. Leads 176, 177 extend from each twin searchcoil assembly 110, 111 through the housing 156 and are connected with a signalling circuit, shown in block diagram in FIG. 15, to be described subsequently. A protective facing, such as a nylon insert 179 180 is positioned directly beneath each pair of twin search-coils. The facing spaces the search coils a predetermined distance from the surface of the billet being inspected and also provides a surface of relatively low friction for riding upon the surface of the billet during the inspection. Excessive wear of the facing is prevented by wear plates 181, 182 that encircle the twin search-coils and the nylon inserts.

' The two proximity coils 112, 1 13 (see FIG. 9) are carried within the housing 156, adjacent the bottom surface thereof along one longitudinally extending side that is adjacent the arm 144 of the mounting bracket 136. Each proximity coil 112, 113 is mounted in a pivoted arm 184, 185, respectively. The arms 184, 185 pivot about a common mounting shaft 188 that extends transversely of the housing 156, in general alignment with the two Search coil assemblies 110, 111. Thearm 184 extends forwardly of the mounting shaft 188, and the arm 185 ex-' tends rearwardly. The proximity coils 112, 113 extend upward from the mounting arms 184, 185, through, en-

larged apertures 190, 192 in the housing 156. Theem larged size of the apertures 190, 192 permits thepivoted arms 1'84, 185 to rotate through a relatively small are about the mounting shaft 188. Spring plungers 194, 195 are secured in the housing 156 and cooperate with the distal ends of the pivoted arms 184, 185, respectively. Thus, the proximity coils 112, 113;are resiliently biased against the surface of the billet being scanned by .the housing 156 and, to a limited degree, follow the surface contour of the billet independently of the housing. This prevents the proximity coils from disturbing the search coils in response to surface irregularities that are beneath the proximity coils but not beneath the search coils. The lower end of each proximity coil 11 2, 113 is positioned at the lower surface of the pivoted. arms 184, 185, and the coils are electrically connected with a proximity sensing system to be described in more detail below. With this arrangement, a proximity coil :is positioned in advance of and to the rear of the twin search coil assemblies 110, 111 during reciprocation'of the scanning unit 165. Thus, in either direction of reciprocation of the scanning unit, one of the proximity coils 112, 113 will be moved beyond the longitudinally extending edge of a billet before the search coils themselves reach the edge. This provides a signal that reverses the control valve 126 to reverse the direction of movement of the scanning unit.

Two nozzles or paint markers 114, 115 are carried by thescanning unit 105 adjacent the opposite side of the housing 156 from the proximity coils 112, 113. See FIGS. 5 and 7. The paint markers 114, 115 are positioned to direct a spray of paint to a localized area on the billet being inspected. As with the proximity coils, one paint marker is located on either side of the axis of alignmentof the twin search-coil assemblies 110, 111, so that one of the markers will follow the search coils in each direction of reciprocation, in a manner similar to the way in which one provimity coil leads the search coils during reciprocation. Depending upon the direction in which the search coils are being reciprocated, one of the paint markers 114, 115 (i.e., the one following the search coils in the direction of movement) will be actuated after a short delay each time the search coils detect a defect. The construction and operation of the paint spray guns, will become better understood in Part III hereof. In addition, suitable paint spray-apparatus for use with this invention is disclosed and claimed in the copending application of Joseph M. Mandula et al., Ser. No. 287,987, filed June 14, 1963, now Par. No. 3,418,567 and entitled High Speed Marking Sys- (C) Calibrating mechanism Reference is now made to FIGS. 3 and 4, where the inspection station 22 is shown, with parts removed, showin the construction and arrangement of the calibrating mechanism. A path of travel P is indicated by a dot-dash line extending between the two spaced conveying rolls 24, 25 and is positioned along the axis corresponding to the center line of a three inch square bil'let. The center lines of the scanning units 45, 46 are indicated at CL, CL respectively. A calibrating bar 205 is located between the two conveying rolls 24, 25, aligned with the path of travel P, of the billet. The calibrating bar 205 is shown in solid line in a lowered position beneath the path of bil'let travel and in phantom lines in a raised position coincident with the path of billet travel. A frame 208 provides a support for the calibrating bar 205. The calibrating bar 205 is connected to the frame 208 by a pair of links at each end of the bar 205. A first pair of links 210 are pivotally connected between a depending lug 212 at one end of the bar ,205 and an upstanding lu-g 215 on the frame 208. Similarly, the opposite end of the calibrating bar 205 is pivotally connected by a pair of links 218 between a depending lug 219 on the bar 205 and an upstanding lug 221 on the base 208.

A third depending lug 225 is located on the calibrating bar 205 mid-way between the two lugs 212, 219. A clevis 228 pivotally connects this lug With a pistonrod 230 of a pneumatic cylinder 231. The pneumatic cylinder is connected at acentra'l pivot 233 to the supporting frame 208.

With the above construction, air supplied to the cylindcr 231 raises or lowers the calibrating bar 205. The bar swings between the lower and upper position on the links 210 and 218. The calibrating bar is maintained in its 'lower position when a billet is being inspected. In the absence of a billet, the calibrating bar may be raised 'by the actuation of pneumatic cylinder 231 to a position normally occupied by a billet. With the calibrating bar in raised position, the two inspection assemblies 45, 46 may be lowered into an inspection position relative to the calibrating bar 205, and the scanning units may be adjusted to a desired sensitivity in accordance with a known defect present in the calibrating bar. The manner of controlling and actuating the pneumatic cylinder for raising and lowcring the calibrating bar will be explained in more detail in Part III.

(III) coNrsdL SYSTEMS The general mechanical arrangement just described is automatically operated by electrical, hydraulic and pneumatic control systems. An overall electrical control circuit for automatic operation and for calibrating the scanning units is provided. Additional electrical circuits include a proximity sensing system with two transducer coils for sensing the edges of the billet being inspected, and also a signalling circuit for detecting defects present in the billet being inspected and for actuating defect markers. The hydraulic control system is actuated in response to the electrical controls and moves the support arms and scanning units to different positions at controlled speeds during an operating cycle. The pneumatic syste rn moves the search coils into an out of proximity with the billet being scanned, raises the calibrating 'bar, and also controls and operates the paint markers for marking defects on the billet. The circuits and systems will be described in conn'ection with-a single inspection station and single scanning unit unless'otherwise indicated, since all operate in (A) Electrical control system The main electrical control circuit is schematically shown in FIG. 12. Three power lines L1, L2, L3 are supplied with 440 volt alternating current. Each line includes one contact of a disconnect switch DSW and two fuses F1, F4; F2, F5; and F3, F6. A normally open contact HP1-1, HP1-2, and HP1-3, respectively, is located in each line L1, L2, L3 between the fuses F4, F5, F6 and a pump motor PM1. The pump motor'PMl supplies power to the hydraulic pumps of one inspection station, e.g., inspection station 22. Power lines L4, L5, L6, connected respectively with lines L1, L2, L3, supply power to a second pump motor PM2 for the other inspection station. The primary winding of a three kva. transformer TFR is connected by lines L7, L8 to power 'lines L1, L2, respectively. The secondary of the transformer is in an electrical line L10 of the control circuit.

(1) Supplying power.-When the disconnect switch DSW is closed to the on" position, 440 volt current is supplied to one side of the contacts HP1-1, 2, 3. These contacts, when closed, will start the pump motor PM1. Current is also introduced to the primary of transformer TFR, which reduces the voltage from 440 across the primary to 110 volts across the secondary, in line L10. As shown in the schematic ladder diagram of the control circuit, line L10 is connected across lines L11, L12, and L13 and is grounded at G. Line L11 contains a fuse F7 and line L12 contains a fuse F8. Lines L14 through L38 extend across lines L12 and L13, and lines L39 through L42 extend across lines L11 and L13.

A mechanical air-pressure switch ASW, a power off switch POS, a power on reset switch PRS and a power control relay CRM are connected in series in line L14. The air pressure switch ASW is set to only close when the air pressure in the pneumatic system is 60 pounds per square inch (p.s.i.) or above to provide protection for the unit, since many parts of the system operate only on air pressure from 60 to 70 p.s.i.

With the power off switch P08 in its normally closed position, the closing of the power on reset switch PRS energizes the power control relay CRM and also the indicator lamp IL in line L16. Energization of the relay CRM closes normally open contacts CRM-1 in line L15 and normally open contacts CRM-2 and CRM-3 in lines L12 and L13, respectively. Contact CRM-1 establishes a holding circuit for relay CRM, and contacts CRM-2, 3 connect the remainder of the control circuit between lines L12 and L13 with power line L10.

(2) Starting pump motors.-Each hydraulic pump motor PM1 and PM2 operates two pumps. PM1 operates pumps for the two scanning heads for one inspection station and PM2 operates the pumps for the scanning heads of the other inspection station. Each pumping unit has a 90 gallon tank and is equipped with mechanical oil level switches OLSl and OLS2. The purposes of these switches is to stop the pump automatically if the oil level is too low or too high. When the oil level is at the proper height, hydraulic pump motor PM1 is started by pressing the start switch HPS in line L18. This completes a circuit through a normally closed hydraulic stop switch HSS, the now closed start switch HPS, and the closed oil level switches OLSl, and OLS2 to energize a hydraulic pump relay HP1. Energization of relay HP1 closes normally open contacts HP1-1,22, 3, thereby connecting the'hydraulic pump motor PM1 to the source of power. Normally open contact HP1-4 in line L19 is closed, establishing a holding circuit for relay coil HP1 and contact HP1-5 in line L22 is also closed. At the same time relay coil HP1 is energized, a timer T in line L20 is energized to record the running time of the pumps. A pump indicator light 1L2 in line L21 is also turned on to indicate that the pump PM1 is running. The operation for starting pump PM2 is identical.

(3) Activating the automatic controL-A Manual-Off- Automatic switch MOA having three positions and several sets of contacts is provided, with contacts in lines L22, L34 and L38 of FIG. 12, as well as in line L50 of the circuit of FIG. 13. An x on the line of the circuit to the side of the contact of the three position switch indicates the position in which the switch is operational. A second three position Manual-Otf-Automfltie switch MOAZ is in line L22. Both switches must be in automatic position for automatic operation. A pendant, which is a portable pushbutton s'tation used for calibrating the scanning heads, connects to line L22 at receptacles PR1 and PR2. The pendant must be plugged into the control circuit to complete the circuit of line L22 for automatic operation, With both switches MOA and MOA2 in automatic position and with the pendant connected, a circuit is provided through the two switches, the pendant receptacles, Closed contacts HP1-5 and HPZ-S (which was closed in the same manner as HP1-5 by the energization of hydraulic power motor PM2) and the ready to run light IL3 is on.

At the same time, current is introduced to one side of an automatic start button ASE through lines L23, L24, three normally closed emergency stop buttons ESl, BS2, BS3, and a normally closed automatic stop button AS. On the other side of the automatic start button ASB, completing the circuit of line L24, are a normally closed contact CR8-1, a relay coil CRA and four safety relay contacts SCI, 8C2, SC3, and SC4. These safety relay contacts are in the photoelectric units and are closed when the units are operating properly. Pressing the automatic start button ASB energizes relay CRA through the circuit of line L24, thereby closing contacts CRA-1 in line L25 to establish a holding circuit and to energize the automatic" indicator light 1L4 in line L26. Relay coil CRA also closes contacts CRA-2 in line L27, energizing coil CRT to set up a circuit in line L29 by closing contacts CRT-1 which will subsequently make available a timer and counter T1 and C1 to record scanning time and raise and lower cycles of the scanning head. The scanning units are now ready for automatic operation.

(4) Lowering scanning head onto billet.There are three photocells associated with each scanning head. Photocells 60, 61 and 62 associated with scanning assembly 45 have been previously described. The photocells have, respectively, contacts PE-1, PE-2, in line L28, and PE-3 in line L38.

The energization of coil relay CRA, previously described, closes contacts CRA-3 in line L28, supplying current to one side of the photoelectric cell contact PE-l via normally closed toggle switch TS. The toggle switch TS allows one head to be raised while the other heads are in automatic operation.

The front of the approaching billet intercepts the beam on the first photocell, closing contacts PE-l. Further movement of the billet breaks the beam on the second photocell, closing contact PE-2 in series with contacts CRA-3, PE-l and toggle switch TS. The two photocell contacts provide a safety feature in the event one is closed by a false signal. This energizes a coil relay CR1 in line L28. This closes normally open contacts CR1-1 in line L39, energizing a fast approach solenoid SOL-1. The fast approach solenoid controls a valve in the hydraulic system shown in FIG. 16 which actuates the hydraulic cylinder 86 that controls the raising and lowering of the support arm 49 for the scanning unit. The energization of solenoid SOL-1 starts the scanning head moving toward the billet.

At the same time, contacts CR1-2 in line L29 are also closed. This completes a circuit through now closed contacts CRT-1 to start the timer T1 in line L29 and the counter C1 in line L30, which record the scanning time and raise, lower cycles.

As the scanning head nears the billet, the sensing roller 103 and lever arm 104 activate a limit switch LS-l, shown with the contact in line L31, energizing coil relay CR2.

This closes contacts CR2-1 completing a holding circuit across the limit switch LS1, assuring that the coil relay CR2 will remain energized. Contacts CR2-2 in line L40 are also closed, thereby energizing a slow-down solenoid SOL-2. This solenoid actuates a valve in the hydraulic system, which controls the movement of the head toward the billet. The movement is restricted so the head does not impact against the billet with a large force. At the same time, normally closed contacts CR2-3 in line L33 are opened, preventing relay CR8 from subsequently becoming energized.

As the scanning head contacts the billet with roller 99, a second mechanical limit switch LS2 with contacts shown in line 34 is activated by the same cam lever arm 104. This energizes coil relay CR3 in line L34 by completing a circuit through a contact of the switch MOA, and the now closed contacts CRA-4 and CR1-3, which were closed when their associated relays were activated. This also completes a circuit through now closed contacts CR1-4 in line L36, energizing time-delay relay coil TRl. When coil TR1 is energized, contacts TRl-l in line L35 are'closed, establishing a holding circuit across the limit switch LS-2. Contacts TR1-2 in line L42 are also closed, energizing the raise head solenoid SOL-3. This serves to reduce the contact pressure of the scanning head on the billet during scanning. The scanning head is not raised from contact with the billet at this time due to the construction of the hydraulic cylinder in the hydraulic system, which will be explained subsequently. The contacts TR1-3 in line L33 are also closed, but contacts CR2-3 have pre viously been opened. It will be apparent from the circuit that when relay CR8 is energized, relay CRA drops out, and the automatic operation is prevented.

The continued movement of the billet intercepts the light beam of the third photocell, closing the contacts PE 3 in line L38. This completes a circuit in line L38, through the closed contact of the switch MOA, the closed contacts PE-3, the closed contacts CR3-l (which were closed when relay coil CR3 in line L34 was energized) thereby energizing the relay coil CR4/Relay coil CR4 closes contacts CR4-1 in line L41, energizing a solenoid SOL-4, which activatesthe air cylinder 116 that lowers the scanning coils to the billet.

The scanning now begins, and will be described in detail in connection with the proximity sensing system and the hydraulic system;

(5') Safety limit switch.-A safety limit switch LS-9 is provided making contact in line L37 and actuated by the support arm 49 that mounts the scanning unit. The purpose of thisi switch is to guard against a premature energization of coil relay CR1.

Coil relay CR1 initiates the fast approach of the support arm toward the billet. If a billet is not present, movement of the arm 49 closes limit switch LS-9. When this happens, relay TRl is energized through the three positions which MGA, the limit switch LS-9, the normally closed contacts 'CR2-4 in line L37, and the now closed contacts CR1-4 in line L36. Energization of relay TRl closes contacts TR1-3 in line L33, completing a circuit of line L33 through the normally closed contacts CR2-3, the closed photoelectric reset switch PRS to energize relay coil'CR8: When CR8 is energized, the normally closed contacts CR8-1 in line L24 are opened and relay coil CRA is de-energized. This opens contacts CRA-3 in the circuit of coil CR1, line L28, thereby de-energizing CR1. The fast approach of arm 70 then stops. Contacts CR1-4 in line L36 open, to de-energize coil TRl. However, TR1 is a time delay relay which holds in from one to two seconds after it is de-energized. As a result, the raise head solenoid SOL-3 is energized even after the fast approach solenoid SOL-1 is de-energized as a result of coil relay CR1 dropping out. This reverses the movement of the arm and the scanning head returns to raised position. With this arrangement, the scanning head is protected 7 numerals. Where circuit lines of this diagram coincide only in part with lines. of the control circuit of FIG. 12, a dot is provided on the line indicating the juncture of a new line and the discontinuation of the indicated line on one side of the dot, which coincides with the line of the control circuit diagram.

A pendant or portable pushbutton station may be plugged into a receptacle associated with each scanning head to calibrate each head individually. The pendant has the following six control buttons:

Calibration bar up Calibration bar down Lower head Raise head Lower search coils Raise search coils When the pendant is plugged in, all pendant receptacles marked schematically by chevrons in the control circuit and calibration circuit are closed to complete the applicable circuit.

Before starting calibration the three position switch MOA is placed in the manual position. By pressing a calibration bar up switch CBU a circuit is completed along line L50, through the switch MOA, the normally closed contact CR8-2, and a closed calibration-bar-down switch CBD to energize a coil relay CR17. This closes contacts CR17-1 in line L51 establishing a holding circuit across the switch CBU. Coil relay CR17 also closes contacts CR17-2 in line L43, thereby energizing a calibration bar solenoid SOL-5. This causes the calibration bar 205 to be raised into calibrating position by the pneumatic cylinder 231.

A lower head" button LHB in line L52 is then pressed, establishing a circuit through a second set of contacts CBD of the calibration bar down switch CBD, a closed raise head button RHB to energize coil relay CR1 in connected line L28. Energization of coil relay CR1 closes contacts CR1-5 in line L53, establishing a holding circuit across the button LHB. Contacts CR1-1 in line L39 are also closed, energizing the fast approach solenoid SOL-1 causing the arm 70 and scanning head to rapidly approach the calibrating bar. Theenergizing of the fast approach solenoid SOL-1 and also the slow down solenoid SOL-2 (through contacts CR1-2 and limit switch LS-l in lines L29 and L31) is the same as explained in the automatic scanning operation, with the exception that activating limit switch LS-2 does not lower the search coils automatically.

The normally open contacts CRA-4 in line L34 close only when the unit is set for automatic operation, thus preventing coil relays CR3 and CR4 from being energized. This gives the operator separate control over the lowering of the head and lowering the search coils.

The search coils are lowered by pressing the lower search coil button LSC in line L54 to complete a circuit through the closed raise search coil button RSC and the now closed contacts CR1-3 to energize coil relay CR3 in line L34 and to also energize coil relay CR4 in line L38 via line L55. This closes contacts CR3-2 in line L56 to hold relays CR3 and CR4 energized. At the same 'time, contacts CR4-1 in line L41 are closed, energizing the search coils solenoid SOL-'4. This lowers the search coils to the calibrating bar. Scanning now begins and the sensitivity of the electronic equipment is set to the required standards. It is essential that the calibrating bar be positioned in an operative position so that the scanning units can move across the bar during calibration. Otherwise the signalling circuit cancels the defect reading and prop er adjustments cannot be made.

When the calibration is completed, the calibrating bar is lowered and the scanning head is raised. This is accomplished by pressing the calibration bar down button CBD in line L50, which also actuates the button SBD' in line 1.52, as indicated by the dotted line. This deenergizes relays CR17 and CR1. As a result, contacts CR17-2 in line L43 are opened, deenergizing the calibration bar solenoid SOL5, and the calibration bar is lowered. The de-energization of the relay CR1 opens the associated contacts, thereby de-energizing coil relays CR2, CR3, and TRl. The tie-energizing of CR3 then deenergizes relay CR4 by releasing the hold circuit of line L56. As already mentioned, relay TRl is a time delay relay which stays energized long enough to permit the hydraulic system to raise the scanning heads from operating position.

It will be apparent that the raise search coils, raise head and calibration bar down buttons can be operated individually if need be. The operation of the safety limit switch LS9 is the same as described in the automatic operation.

(7) The proximity sensing system.The direction of movement of the scanning unit 105 is controlled by the valve 126. In turn, the valve is controlled by the proximity sensing system, which senses the edge of the billet being scanned and shifts the valve, reversing the direction of movement of the scanning unit. The two proximity coils or transducer coils 112, 113 and a shielded cable associated with each, provide the necessary inductance-capacitance ratio for the resonant circuit of a conventional Hartly Electron-Coupled Oscillator, which oscillates at a frequency of 450 kc. The proximity circuit is shown diagrammatically in FIG. 14.

Two proximity coils PC1 and P02 are shown in FIG. 14, each having its own oscillator V1 and V2, respectively, the capacitance of the cable for each coil is shown in phantom at CA1 and CA2. Briefly, the output of these two oscillators is capacitive coupled through germanium diodes D1 and D2, respectively, to the grids of vacuum tubes V3A and V3B, respectively, causing the tubes to conduct current.

As tubes V3A and V3B start to conduct, the plate voltages drop from 135 volts to 55 volts. This voltage is diode coupled through diodes D4 and D5 to the plates of tubes V4A and V4B, then through a voltage dividing network to the grids of tubes VSA and VSB. The output of tubes VSA and VSB is coupled to tubes V6 and V7, respectively, through a voltage divider network, which is returned to the negative 150.volt power supply. The voltage divider network maintains a negative voltage of 37 volts on the grids of tubes V6 and V7 and biases them into cut-01f.

The proximity coils PC1 and PC2 oscillate when they are not adjacent the metal billet and cease to oscillate when they sense the metal. When the scanning head comes down on a billet, the top proximity coil PC1 is positioned to sense the metal billet. At this time, oscillation ceases and there is no output from tube V1. This means that no signal is being fed to the grid of the tube V3A. Without this signal, the voltage normally set up by a Zener diode ZD biases the tube V3A into cutoff. When this happens, the voltage at point P1 rises from 55 volts to 135 volts.

This allows the voltage at point P2 to also rise to 135 volts. This increase in voltage also increases the voltage at points P3 and P4.

The voltage increase at point P3 causes tube V4B to conduct heavily, decreasing the voltage at point P5 and reverse biasing diode D5.

The voltage increase at point P4 causes tube VSA to conduct, increasing the voltage at point P6. This increased voltage at point P6 then overcomes a negative 37 volts that exists at point P7 when no metal is sensed 16 by the proximity coil PC1, causing the tube V6 to conduct.

When tube V6 starts to conduct, it energizes a solenoid SOL-P, which operates the control valve 126, starting the scanning assembly to move in an upward direction.

As the scanning assembly moves upward across the surface of the billet, the bottom proximity coil comes into overlying relationship with the billet and senses the metal.

This stops the oscillation of tube V2.

There is now no output from the tube V2 and, therefore, no signal is being fed to the grid of tube V3B. The voltage at point P8 now rises to 135 volts, but diode D5 cannot conduct current because of the low voltage at point PS. This low voltage at point P5 is caused by the heavy current being conducted by tube V4B. This effect is called the reverse biasing of diode D5.

The scanning assembly continues in its movement upward across the surface of the billet until the top proximity coil PC1 passes beyond the top edge of the billet. AI. this time, the coil no longer senses metal and tube V1 starts to oscillate.

The output of tube V1 to the grid of tube V3A again causes V3A to conduct. Point P1 now falls to 55 volts and point P2 also drops to the same voltage. Point P4 also has decreased voltage, causing point P6 to decrease in voltage also. There is now insufiicient voltage to overcome the negative voltage (minus 37 volts) at point P7 and tube V6 is now cut-off. This de-energizes the valve solenoid SOL-P, reversing the control valve 126 and the scanning assembly starts to move downward.

At the same time the scanning assembly movement reverses, the voltage at point P3 also decreases, causing the voltage at point P5 to increase. Diode D5, then, is no longer reversed biased and the voltage increases to 135 volts, the same voltage as at point P8. This voltage increase also occurs then at points P9 and P10.

When the voltage at point P10 increases, it causes tube VSB to conduct heavily, and this increases the voltage at point P11. This increased voltage then overcomes a negative 37 volts that exists at point P12 of tube V7 when no metal is sensed by the proximity coil PC2, causing V7 to conduct. The conducting of tube V7 energizes a relay MR, which reverses the operation of the defect markers 114, for the downward scanning movement. The operation of the markers will be described below.

The increasing voltage at point P9 causes the tube V4A to conduct heavily, decreasing the voltage at point P2. This then, puts a reverse bias on diode D4.

As the top proximity coil PC1 then comes back over the billet, tube V1 will stop oscillating and the voltage at point P1 will again increase to volts. However. the lower voltage at point P2 stops diode D4 from conducting, thereby preventing the control valve solenoid SOL-P from being energized while the scanning assembly is in its downward movement. The scanning assembly continues its downward motion until the bottom proximity coil goes past the bottom edge of the billet and tube V2 goes into oscillation. The cycle then repeats itself.

The scanning test button TB and leads I 1, J2, and J3, adapted to be connected to a volt meter, are provided to facilitate the adjustment and testing of the proximity circuit.

(8) Signalling circuit.The manner in which the twin search coils of each scanning unit or head operate can be best understood from the diagrammatic block diagram of FIG. 15. A detailed description of the coils is disclosed in the said application Ser. No. 462,907. Two scanning assemblies of one inspection station are represented, and indicated as No. 1 head and No. 2 head. Boxes 250 and 251 represent the coils associated with the No. 1 head and boxes 252 and 253 represent the coils associated with No. 2 head. These boxes 250, 251 correspond with the coils 110, 111 shown structurally in FIG. 

